- Definitions
- What is a Network?
- Network Segment
- Network Interface
- Physical Network Topologies
- Bus
- Ring
- Star or "Hub and Spoke"
- Point to Point
- Point to Multipoint
- Logical Network Topologies
- Peer-to-Peer
- Client - Server
- Distributed Services
- Communication Methods
- Point-to-point
- Broadcast, multiple access (Broadcast
Domain)
- Broadcast, non-multiple access
- Nonbroadcast, multiple access
- Point-to-multipoint
- Network Devices
- Repeaters (Hubs)
- Bridges, Bridging
- Switches, switching
- Routers
Definition of a Network: What is
a Network?
A network is set of
computers linked together for the purpose of communicating and
sharing information. The
Internet
is a global super-network, so is the
local area network ( a
LAN)
at your workplace or your school, as is the wireless hotspot at your local coffee shop,
hotel or library, the
telehone and cellular systems,
and the
satellite communications in space.
What defines a network is often defined by who owns and operates
the equipment and the
computers that
are part of the network. Thus, your school's network is separate from the
Internet.
You know you have a network when you have two or more
computers
connected together and they are able to communicate. Plugged into the back
of each
computer is some sort
of communications port. Nearly all
computers today
have one or more serial ports, parallel ports, Ethernet ports, modem ports,
firewire ports, USB ports and more. All of these ports can be used in
one way or another to connect
computers to
a network. The most common type of network port is an
Ethernet port (the
square port with the row of connectors on the bottom). The next most common
is a
wireless network connection, but that has no physical connector port.
Xerox was the first company to research and develop a network.
Once upon a time, Xerox printers were extremely expensive, so companies wanted
to share them. Xerox knew their
printers were expensive and users were only
able to print from one big
computer (a
mainframe) attached to the printer directly. Xerox decided that they could
sell more printers if they could make it possible for anyone to use the printer
from any
computer. To allow multiple
computers to communicate with the printer, some means of sharing a connection
to the printer was needed. Xerox put Bob Metcalf and others to work on researching
and designing what eventually came to be called
ethernet.
Hosts, End Stations and Workstations
When people talk about networks, they often refer to
computers that
are at the edge of the network as
hosts, end stations, workstations,
or servers.
Its all just the same thing, a
computer attached
to the network; though the word HOST has the most general meaning and can
include
anything
attached to the network including
hubs,
bridges,
switches,
routers,
access points,
firewalls,
workstations, servers, mainframes, printers, scanners, copiers, fax machines
and more!
Just about everything electronic that has a processor and which you would
use in an office is 'network capable' today and lots of things that aren't
currently networked probably
will be networked in the future. In many offices the phone system
already IS the network (
Voice over
IP).
LAN, MAN, WAN and er.. IPAN??
There are some terms, acronyms actually, that are used to describe the size
and scope of a network:
LAN,
WAN, MAN. We've added our own term 'IPAN'
LAN
A
Local Area Network (LAN) is
usually a single set of connected
computers that
are in a single small location such as a room, a floor of a building, or
the whole building.
MAN
A
Metropolitan Area Network (MAN) is a network that encompasses
a city or town. It is usually multiple point-to-point fiber-optic connections
put together by a communications company and leased to their customers,
but a small number of big corporations have built a few of these of their
own and opened them to the local companies with which they do business. The
automotive, travel and insurance industries are just a few examples of who
has built a WAN.
WAN
A
Wide Area Network (WAN) is
usually composed of all the links that connect the buildings of a campus
together, such as at a University or at a corporate headquarters.
WAN connections
can often span miles, so you frequently hear peole referring to the
'WAN' connection
to an office half way around the world. Usually, what distinguishes a WAN
from a LAN is that there are one or more links that span a large distance
over
serial,
T-carrier or
ISDN,
Frame
Relay or
ATM links.
IPAN
So what the heck is an IPAN? An IPAN is an
Inter-Planetary
Area Network. NASA has built a
Deep
Space Internet that uses a
store-and-forward communications protocol called Disruption Tolerant Networking
(DTN). The mechanical rovers
Spirit and
Opportunity on
the planet Mars, have addresses on a NASA network and NASA uses Internet
protocols to communicate with the Mars rovers. While the
communication with the Spirit rover doesn't actually get transmitted over
the Internet, the NASA network does have hosts spanning between the planets
Earth and Mars. They also have probes they have sent into the outer solar
system
How does a network work?
From a very practical and minimalist standpoint, we can start with the most
common type of network, an Ethernet based local area network as an example.
The network is built with several layers of technologies piled on top of
one another.
- Ethernet
provides
- Physical communication using electricity over copper wiring
- Physical
addresses (Media Access Control or MAC addresses)
- Dynamic Host Control Protocol - DHCP
- Provides a logical network address (an IP address)
- Address Resolution Protocol / Reverse Address Resolution Protocol
provide
- Allows computers to announce a logical address (IP address)
they are looking for and to discover which physical address (MAC
address) is using that address.
- Internet Protocol
- Logical Addressing
- Routing
- Transmission Control Protocol or User Datagram Protocol
- Domain Name Service (DNS)
- Host name and IP Address resolution
- Other services and protocols (such as HTTP which
makes the World Wide Web possible)
All networks have several layers of functions
stacked on top of each other. Ethernet
is used to provide the means to transmit information encoded in electrical
signals across copper wiring between two computers. Internet Protocol networking
software running on the computers use the Ethernet network to send data
back and forth inside
IP
packets. The
Internet
Protocol layer provides the
means for the computer to connect to the network, obtain a
logical
address,
to learn the logical addresses of other computers and to communicate with
the other computers on the network.
Internet
Protocol provides the basic
network functions.
Physical Network Topologies
The hardware used to build the network will usually require that the structure
of the network conform to a certain design. The word
topology is
used to describe what the network looks like when drawn on paper and to a
large extent, how it operates.
- Bus Topology
- A bus topology connects all computers
together using a single wire, usually a piece of coaxial cable, that
passes electricity over a copper core that all devices transmit and
receive from. All devices hear all communication over the bus.
- Ring Topology
- A ring topology usually involves connecting one or more computers
together using paired physical interfaces. One interface is the
clockwise side of the ring, the other connection is the
counter-clockwise side of the ring. Devices connected to the ring can
transmit and receive, but there is usually some other sort of method for
controlling access to the common network hardware. Token
Ring uses a ring topology as does CDDI and FDDI. All three of these network technologies use a token-passing scheme in which the computer holding the the token is allowed to transmit.
- Star Topology
- A star topology
is the most common network topology in use today. All devices in the
network are connected to a single hub or repeater. The connected devices
radiate outward from the hub like an asterisk '*' or star.
- Hub and Spoke Topology
- Hub and spoke is another term often used to describe a star topology.
- Point to Point Topology (Daisy Chaining)
- A point-to-point topology
is most often a communications connection between two devices over a
single hardware connection that is not shared by any other devices.
There will be exactly two and only two devices on the connection.
Networks using point-to-point topologies can be daisy-chained together to form an end-to-end communications path.
-
- Point to Multipoint
- A single connection point on the network has network segments that run to several other points.
-
-
Logical Network Topologies
Peer-to-Peer
A
peer-to-peer network is composed of two or more self-sufficient computers. Each
computer handles all functions, logging in, storage, providing a user interface etc. The
computers on a peer-to-peer network can communicate, but do not need the resources or services available from the other
computers on the network. Peer-to-peer is the opposite of the client-server logical
network
model.
A Microsoft Windows Workgroup is one example of a peer-to-peer network.
UNIX servers
running as stand-alone systems are also a peer-to-peer network. Logins,
services and files are local to the computer. You can only access
resources on other peer
computers if you have logins on the peer computers.
Client - Server
The simplest
client-server network is composed of a server and one or more clients. The server provides a service that the client
computer needs. Clients connect to the server across the network in order to access the service. A server can be a piece of
software running on a
computer, or it can be the
computer itself.
One of the simplest examples of client-server is a
File Transfer Protocol (FTP) session.
File Transfer Protocol (FTP) is a protocol and service that allows your
computer to
get or
put files to a second
computer using a network connection. A
computer running
FTP software opens a session to an
FTP server to download or upload a file. The
FTP
server is providing file storage services over the network. Because it
is providing file storage services, it is said to be a 'file server'. A
client software application is required to access the
FTP service running on the file server.
Most
computer networks today control logins on all machines from a centralized logon server. When you sit down to a
computer and type in your username and password, your username and password are sent by the
computer to the logon server.
UNIX servers use
NIS,
NIS+ or LDAP to provide these login services.
Microsoft
Windows comptuers use
Active
Directory and Windows Logon and/or an LDAP client.
Users on a client-server network will usually only need one login to access resources on the network.
Distributed Services
Computer networks using distributed services provide those services
to client computers, but not from a centralized server. The services are
running on more than one
computer and some or all of the functions provided by the service are provided by more than one server.
The simplest example of a distributed service is
Domain
Name Service (
DNS) which performs
the function of turning human-understandable
domain
names into numerical
(dotted quad)
computer addresses called
IP addresses. Whenever
you browse a
web
page, your
computer uses
DNS. Your
computer sends
a
DNS request to your local
DNS
server. Your local
DNS
server will then contact a remote server on
the Internet called a "
DNS
Root Server" to begin
the lookup process. This
DNS
Root Server will then direct your local DNS server
to the owner of the
domain
name the
website is a part of. Thus, there are
at least three DNS servers involved in the process of finding and providing
the
IP
address of the
website you intended to
browse. Your local DNS server
provides the query functions and asks other servers for information. The
Root DNS server tells your local DNS server where to find an answer. The
DNS server that 'owns' the domain of the website you are trying to browse
tells your local DNS server the correct
IP
address. Your
computer stores
that
IP
address in its own local
DNS cache.
Thus, DNS is a distributed service that runs everywhere, but no one
computer can
do the job by itself.
Network Devices
Today's network environment is predominently Ethernet technologies. Ethernet
is a broadcast protocol that provides the
physical
layer and
data-link
layer functions
within a network. To connect devices that use ethernet, you need a hub,
bridge,
switch or
router, and which device you use depends on how you will use the
network and the computers.
Hubs
A hub is a device used to create a
broadcast
domain so that several
computers can communicate. Hubs are very inexpensive.
Hubs receive frames and broadcast the frame on all ports. Hubs are dying out
of existence and the only place you see them today is in extremely small networks,
especially in homes. Once there are too many computers connected to a hub,
communication begins to break down and a more intelligent solution is required.
Bridges
A bridge is a smarter version of a hub, and performs the same function. A
learning bridge can figure out where a given computer is located and transmit
frames only on the port connected to that device.
Switches
A switch is a specialized type of learning bridge that can learn which devices
are connected to which ports and can forward frames only to those ports that
are supposed to hear the transmission. Bridges maintain a forwarding table.
Switches are the most common way in which ethernet networks are wired together.
Routers
Routers break up broadcast domains and segment networks, which allow network
administrators to control broadcasts and control access to various network
resources. Routers provide a means to allow computers to share a logical network
that is separate from the physical switched network. Routers are used to connect
several ethernet networks together to make a larger network. Routers can further
extend the network by connecting local networks to the global network called
the "Internet".
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